The invention is directed to a process of consolidating soil as well as the frost protective layers produced thereby as subsoil or substructure for roads and railroad construction.
In building roads and railroad lines customarily before applying the road surface (top of the road) or applying the road bed in railroad construction the subsoil or substructure is solidified. Today this occurs according to a process known as soil consolidation in which the different soils (loose compositions) as for example soil according to DIN 18196 (German Industrial Standard 18196), pulverulent mineral materials or mixtures of the same are treated with water and cement, for example blended with soil pulverizers or in mixing plants and then are compacted by the action of rolls, for example by means of rubberized rolls. Through the hardening of the cement contained in the soil consolidating composition the individual particles of the soil consolidating composition are united to a solid cemented framework. While the particles are practically completely enclosed in the cement stone of concrete, in soil consolidation the particles are cemented only at individual points. Hence very much higher compression strengths are sought with concrete and correspondingly more cement is added than is necessary with soil consolidation where depending on the soil one manages with 80 to 220 kg of cement per m.sup.3 (corresponding to 4 to 16% cement). Under the above mentioned pulverulent mineral materials which below and in the patent claims for short are designated by the term "rubbish" there is understood natural and synthetic mineral materials such as fly ash, combustion residues, other pulverulent or fine sand containing residues from dry, wet and electro dust removing plants, silt and clay containing wash residues from grit and quarry stone plants, rubbish materials from grinding processes and other finely divided inorganic and organic residues of any type.
Because of the different purposes and the different materials employed there are basic technological differences between the consolidation of soil with cement and the manufacture of concrete. While with concrete (cement concrete) there is always assumed a practically complete compression so that the hollow spaces between the individual grit and sand particles are nearly completely filled with cement glue, in the soil consolidation there is not produced a practically complete compression.
Therefore with concrete (except for the quality of the cement) the quality is determined by the cement glue, i.e. the water cement value and the porosity of the hardened cement stone dependent thereon. In contrast with soil solidification there cannot be placed any particular requirements aiming at a minimum of hollow space. Even with good compression therefore in soil solidification there remains in the particle framework more hollow space than in concrete. While concrete usually strives for a residual pore or void content of approximately 2.0 volume % or less and only in exceptional cases as in road concrete seeks a total pore content of about 4 vol %, the pore or void portion in soil solidification is 10 to 20 times greater.
The production of compositions for the solidification of soil with cement therefore in contrast to the production of concrete takes place according to other principles, namely those of soil mechanics. This starts from a system of solids as well as water and air as the voids. Basically there is attempted to attain a highest arrangement of mineral material according to the rule: the larger a mass per unit of volume, the more is the resistance to deformation. Accordingly the essential determining factors for the quality of soil solidification with cement are the water content, the cement content and the extent of compression.
There can be used for the solidification of soil with cement any of the soils occurring in nature which can be comminuted to the required degree, do not contain any materials disturbing hardening and are miscible with cement (hydrophobized or not hydrophobized) and water as well as in a given case suitable additives. Thereby the particle size distribution of the soil to be solidified follows a course far above (or outside) the sieve lines noted for cement concrete (according to DIN 1045). Water and cement behave here in no water cement value ratios similar tot hose of concrete technology.
Accordingly there is no possibility to calculate "steady" fixed tensile strength properties in the soil solidification with cement.
As has already been mentioned above the consolidation can take place with the mixing in of rubbish or, as already carried out experimentally with exclusive use of rubbish such as fly ash. In this case there is likewise valid essentially the previously mentioned point of view.
The water in the solidified soil-cement mixture acts as "lubricant". Accordingly there is for each soil, rubbish, or soil/rubbish mixture or for each soil-cement mixture, soil-rubbish-cement mixture or rubbish-cement mixture from the viewpoint of the above mathematical interrelationship a so-called "optimal water content", which is ascertained in the so-called Proctor experiment (see the pamphlet DIN 18127 for the Proctor experiment, published by the Forschungsgesellschaft fur das Strassenwesen). Hereby on principle there is employed dry soil-cement mixture (or the other mixtures previously mentioned), to which there is added increasing amounts of water. Each mineral-water mixture is then struck with fixed blows of a normalized compression hammer in the Proctor pot. Each experiment permits the determination for each Proctor pot charge a so-called moist space density. After the determination of moisture there is calculated from the moist space density a dry space density whereby the highest dry space density is calculated at the optimal water content. This can be determined in most cases with about five individual experiments. If there is plotted the determined dry space densities (ordinate) against the water contents corresponding thereto then there is frequently attained a curve similar to the Gaussian distribution. There can be derived from this type of curve that for the production of the highest dry-space density in each case considering a compression energy in Proctor pot of about 0.6 Mn/m.sup.3, there is a specific water content.
Since there are no sieve lines similar to the concrete fcr consolidating soil the mineral hollow space in the Proctor process is determined in the manner where the highest dry space-density for the so-called "raw density" is placed in the ratio. From a dry-space density of e.g. 1.90 kg/dm.sup.3 and a raw density of 2.65 kg/dm.sup.3 there is calculated for the mixture a mineral hollow space of: ##EQU1##
While a concrete, as already mentioned, merely has about 2.0 vol. % of pores or voids the mineral hollow space for soil consolidation with cement fluctuates in a wide range between about 20 and 40 vol. %.
The water requirement in the soil consolidation, thus corresponds to the "optimal water content", which, as described above, can be ascertained according to the laws of soil mechanics. The dry space density (Proctor density) corresponding to this optimal water content is generally also sought in the design of the structure, assuming that the results of the Proctor test in the associated production of sample cylinders confirm for the determination of the required or suitable cement content for attaining the necessary compression strength.
The above described Proctor test likewise serves for the production and investigation of sample bodies. For this purpose with the previously ascertained optimal water content mixtures of soil, cement and water were produced in such manner that the cement content generally is varied in three steps, e.g. 5%, 7% and 9% cement. After the body formed in the Proctor test has been extruded from the mold it is investigated according to specified processes at 7 and/or 28 days after its production fcr compression strength (see TVV 74, Bundesminister fur Verkehr, Abt. Strassenbau, West Germany). Hereby the increase in strength is in somewhat linear relationship with the increase of the cement content. There is connected interpolation from a compression strength which is reached to the cement requirement interrelated therewith (see "Beton" 19 (1969), pages 19 to 24).
There is no relationship in soil consolidation between strength properties and water cement value as is the case with concrete. The use of specific types of cement and classes of cement material is frequently greatly limited in soil consolidation on account of special concerns. For example because of the desired quick hardening of a soil consolidation customarily there is used a correspondingly quick hardening cement. Preferably there is used as suitable cement for soil solidification normal Portland cement PZ35F (according to DIN 1164) as well as hydrophobized special cements formed therefrom as for example Pectacrete-cement.
As already mentioned the processing of the soil consolidation compositions (construction mixing or central mixing process) thoroughly mixed by soil pulverizers or incorporated by road finishers takes place for example through the action of rolls by means of rubberized rolls. Under a static roll load of about 10 tons (the tons throughout the specification and claims are metric tons) by multiple passages of the rolls the solid particles are compressed to such an extent that the dry density ascertained in the Proctor test is nearly reached or even is partially considerably exceeded. A statement of the consistency in soil consolidation compositions is unnecessary because there are in each case only "damp" and therewith generally drier than a comparable concrete of about the consistency K.sub.1. There is no such thing as a so-called "optimum compression" in soil consolidation as there is with concrete since the degree of compression always conforms to the "Proctor density" determined from the same composition, i.e. the highest dry-space density in the Proctor process.
Although the soil consolidation with cement shows a considerable improvement of the properties of the subsoil or substructure, especially in regard to the resistance to frost in building roads and railroads, this process, however, also has various disadvantages. Through the addition of separate water beyond the true water content of the consolidation material in situ there are required additional process steps and increased expenses. Furthermore there occurs in this type of consolidated soil or frost protection layers so-called macro-cracks in the hardening because of shrinkage. This makes it necessary for example that relatively thick bituminous or cement bound road surfaces must be used in order to avoid the reflection of the macro-cracks in the top of the road. There is a great demand for a process for soil consolidation which can be carried out in a simpler and cheaper manner and leads to the best possible results in regard to resistance to frost, bearing strength and crack structure. Thereby it is especially desired that there be avoided macro-cracks since this would permit the use of thinner bituminous or cement bound road surfaces which in view of the scarcity and increasing price of petroleum in the future presents a continually increasing necessity.
Accordingly the present invention is based on the problem of developing a process for soil consolidation and frost protective layers produced thereby, especially for road and railroad construction, which in contrast to the conventional soil consolidation with cement can be carried out in a simple manner and in a given case using lesser amounts of cement and water. Furthermore the process of the invention should lead to a reduced formation of macro-cracks so that for example in the building of roads there can be used thinner bituminous or cement bound road surfaces.